[Previous] | [Session 37] | [Next]
H. Campins, D.W. McCarthy (U. of Arizona), S.R. Stolovy (Caltech), S.D. Kern (MIT), S.M. Larson (U. of Arizona), N.H. Samarasinha (NOAO), A. Ferro (U. of Arizona)
Comet Hale-Bopp was imaged at several wavelengths from 1.87 to 2.2 microns, using the Hubble Space Telescope's NICMOS instrument. Our post-perihelion observations were obtained over a 1.7 hour period, starting on UT 1997 August 27.953. The comet's heliocentric and geocentric distances were 2.5 AU and 3.0 AU, respectively. Diffraction limited ( < 0.2 arcsecs) images were obtained to probe the the composition and dynamics of the inner coma and the size and activity of the nucleus. An outburst occurred approximately seven hours prior to our NICMOS observations. The dynamical evolution of this outburst was followed over the observing period, and appears similar to a pre-perihelion outburst observed in ground based images (S. M. Larson et al. DPS 2000 abstract). The inner coma (radius r < 2,500 km) is dominated by an arc-like feature, which expanded and became more diffuse with time. Less prominent, time-variable linear and circular features are also apparent; the measured expansion velocities of these features imply a common origin. Several static linear features extend to the edge of the field of view (r = 21,000 km). Modeling of the arc feature in the inner coma requires an outburst from a near-equatorial region in the comet's nucleus, just prior to local noon. Images in a narrow filter, centered at 2.04 microns, exhibit a ~ 2% absorption feature relative to nearly simultaneous images at 1.87, 1.9 and 2.2 microns. This absorption is attributed to water ice in the coma grains. During the observation sequence, the strength of this absorption remained constant, while its spatial extent expands outward. This constant strength of the 2.04 micron absorption suggests that the icy grains did not sublimate significantly during this period. The comet's total integrated brightness within our field of view remained constant. The unresolved nucleus is revealed by a classic point spread function, with a flux density consistent with a 40 km diameter (assuming a 4% geometric albedo). This work was supported by NASA, and is being submitted to Icarus.